1. Field of the Invention
This invention relates to a thickness measuring apparatus for non-metallic sheet-shaped bodies formed of rubber, plastic or the like.
2. Description of the Prior Art
As such kind of thickness measuring apparatus, the following non-contact type thickness measuring apparatus which makes use of radiation, light beam, electrostatic capacity type displacement meter or the like have therefore been proposed.
In the conventional thickness measuring apparatus which makes use of radiation, .beta.-ray, X-ray or the like, radiation is incident upon a non-metallic sheet-shaped body and the thickness thereof is measured on the basis of the rate of attenuation of the amount of radiation transmitted through the sheet-shaped body.
In the conventional thickness measuring apparatus which makes use of light beam, the light beam is incident upon the surface of a non-metallic sheet-shaped body and this surface position is detected on the basis of the light reflected therefrom to measure the thickness thereof.
In the conventional thickness measuring apparatus which makes use of the electrostatic capacity type displacement meter, a change of the electrostatic capacity corresponding to change of the surface position of the non-metallic sheet-shaped body is detected to measure the thickness thereof.
The conventional thickness measuring apparatus which makes use of the radiation, however, has the disadvantage that the apparatus is expensive and is large in size as operators must effectively be prevented from being exposed to the radiation and that if the radiation absorption coefficient of the non-metallic sheet-shaped body is not uniform, the error becomes large, and as a result, it is impossible to accurately measure the thickness thereof.
The conventional thickness measuring apparatus which makes use of the light beam is constructed such that not only the surface position of the non-metallic sheet-shaped body but also the rear surface position thereof are optically detected to measure the thickness of the sheet-shaped body. As a result, the apparatus has the disadvantage that the optical system is complex in construction, that the apparatus as a whole is large in size, and that apparatus takes up a large amount of space. In addition, it is required to define a reference position for the purpose of detecting the upper and lower surface positions of the non-metallic sheet-shaped body. As a result, if the non-metallic sheet-shaped body is considerably displaced in its thickness-wise direction, it is difficult to define the reference position.
The conventional thickness measuring apparatus which makes use of an electrostatic capacity displacement meter has the disadvantage that the measurement error becomes large when temperature changes, so that the thickness could not accurately be measured. Also, it is difficult to define the reference position in the same manner as in the conventional thickness measuring apparatus which makes use of the light beam. As a result, it is impossible to use such conventional thickness measuring apparatus in the case of measuring the thickness of a rubber sheet adapted for use in manufacturing tires at a high temperature.